1. Field of the Invention
The present invention is generally in the field of materials used in semiconductor chips. In particular the present invention is in the field of ceramics and dielectrics used in semiconductor chips.
2. Background Art
It is conventional in the art of fabrication of semiconductor chips to utilize ceramic materials for a variety of purposes. Ceramics can be used as insulators, capacitor dielectrics, barriers to diffusion, or barriers to etching. Semiconductor processing engineers continually search for new and improved non-metallic materials which can be used as insulators, capacitor dielectrics, barriers to diffusion, and barriers to etching. With the advent of copper as the interconnect metal of choice in semiconductor chips, there has come a need to accelerate the search for new and improved non-metallic materials that can be assimilated in fabrication processes which use copper as the interconnect metal.
Conventionally, tantalum nitride is fabricated using a nitrogen content of approximately 21% which results in a metallic tantalum nitride. Tantalum nitride has been used as a metallic barrier layer to prevent diffusion of copper in semiconductor chips. Fabrication of tantalum nitride, as a metal, has been integrated in fabrication processes which use copper as an interconnect metal. However, tantalum nitride has not, to applicants"" knowledge, been fabricated in a ceramic mode. More particularly, a ceramic tantalum nitride has never been used in fabrication processes using copper as an interconnect metal.
Prior to the present invention, there has been no method of taking advantage of the fact that the process of fabricating the metallic tantalum nitride has been integrated into copper processes and that the same integration of the fabrication process could be advantageous when utilizing tantalum nitride in a ceramic mode. In fact, to applicants"" knowledge, prior to the present invention there has been no attempt to use tantalum nitride as a ceramic in semiconductor chips.
Due to ease of integration of the fabrication of tantalum nitride (whether in metallic mode or in ceramic mode) in copper processes, a number of advantages can result from use of tantalum nitride as a ceramic. The entire process of fabricating a ceramic tantalum nitride can be done in the same tool where copper interconnect is fabricated and without a need to xe2x80x9cbreaking vacuum.xe2x80x9d In other words, a layer of the ceramic tantalum nitride could be fabricated as a dielectric above a copper interconnect segment without having to take the semiconductor wafer out of the vacuum chamber for a separate fabrication of the dielectric. The fact that the fabrication of the ceramic tantalum nitride can be accomplished in the same vacuum chamber where copper interconnect is fabricated results in a significant increase in throughput and also reduces risk of contaminating the wafer. In this manner, capacitor structures can be built with higher throughput while reducing the risk of wafer contamination.
Also, the conventional etch stop layer in semiconductor chips has been silicon nitride. Etching various semiconductor chip structures, such as vias, is typically performed using a carbon fluoride based plasma. As such, employing an etch stop layer such as silicon nitride is not ideal since silicon nitride reacts with fluoride. Use of a non-silicon based etch stop layer, such as the ceramic tantalum nitride, can provide a great advantage since ceramic tantalum nitride does not react with fluoride. As such, the ceramic tantalum nitride acts as a stronger etch stop layer than the conventional silicon nitride.
Thus, there is need in the art to consider an alternative ceramic material whose fabrication can be easily assimilated in copper fabrication processes and which also has advantages such as providing a good barrier to etching.
The present invention is a method for fabrication of ceramic tantalum nitride and improved structures based thereon. According to the present invention, an ionized metal plasma (xe2x80x9cIMPxe2x80x9d) tool is used to create a plasma containing tantalum ions where the plasma is sustained by a mixture of nitrogen and argon gases. The percentage of nitrogen partial flow in the mixture of gases is adjusted so as to result in a layer of tantalum nitride with a nitrogen content of at least 30%. With a nitrogen content of at least 30%, the tantalum nitride becomes ceramic.
The ceramic tantalum nitride presents a number of advantages. For example, the fabrication of ceramic tantalum nitride can be easily incorporated into fabrication of semiconductor chips using copper as the interconnect metal. Also, ceramic tantalum nitride can be used as an effective etch stop layer. The reason is that ceramic tantalum nitride does not react with fluoride which is a typical constituent of etchants utilized to etch silicon-based dielectrics such as silicon dioxide.
Further, ceramic tantalum nitride can be used as a dielectric in fabrication of a capacitor stack using copper electrodes. Since fabrication of ceramic tantalum nitride is easily assimilated with fabrication of copper, the capacitor stack utilizing ceramic tantalum nitride can be built in a single IMP tool along with the copper electrodes. The result is higher throughput and a reduced risk of contaminating the semiconductor wafer since there is no need to xe2x80x9cbreak vacuumxe2x80x9d for a separate fabrication of the dielectric layer.